Previously, the majority of semiconductor devices manufactured were those with an Al/SiO2 multilayer interconnect structure, which uses Al, Al alloy or the like as an interconnect material, and an SiO2 film as an interlayer dielectric. In recent years, in order to reduce the interconnect delay caused by the miniaturization of semiconductor devices, semiconductor devices with a Cu/low-k multilayer interconnect structure, which uses Cu (copper) with low resistance as an interconnect material and a low-k film (a low dielectric constant film) with low interconnect capacitance as an interlayer dielectric, have been manufactured in large quantities.
Cu/low-k multilayer interconnect structures are produced by a process called damascene. In dual damascene, which is one type of damascene process, trenches and via holes for an interconnect are first continuously formed by a dry process in an interlayer dielectric substrate made of a low-k film or the like.
A via-first process is one method for forming a dual damascene structure. In this process, via holes are first formed in an interlayer dielectric substrate by dry etching, and then filled with a filling material and planarized. Lithography is subsequently performed to form trenches, and dry etching follows. Ashing or a like process is subsequently performed to remove unwanted substances such as resist or filling material from the interlayer dielectric substrate having trenches and via holes.
Even after this process, however, unwanted substances (hereinafter referred to as “residues after a dry process”) that cannot be completely removed remain on the substrate.
In a damascene structure, when trenches and via holes are filled with metals such as TaN as a barrier metal and Cu as an interconnect material, the presence of residues after a dry process leads to defective semiconductor devices. For this reason, these residues are removed using a residue-removing solution such as a polymer-removing solution.
After removing the residues after a dry process or Cu oxide film, the trenches or via holes are filled with an interconnect material such as Cu. Unwanted Cu portions are then removed by chemical-mechanical polishing (CMP) for planarization to form an interconnect structure. At this moment, metals, particles used for polishing, etc., metal ions, and the like remain on substrate surfaces. A post-CMP cleaning solution is used to remove these residues.
Cu surfaces after a dry process in the formation of a damascene or dual damascene structure have been damaged, and are thus structurally more fragile than the original. Therefore, even if corrosion of the Cu bulk due to the residue-removing process using a polymer-removing solution or the like is not observed, a close examination sometimes reveals Cu surface roughness or cracking along the grain boundary of the Cu surface. It is very likely that these minute changes on the Cu surface adversely affect the device performance.
A crack inhibitor is used in particular cases where cracking would easily occur; however, it may not necessarily provide a sufficient effect. Moreover, some sulfur-containing compounds effective for cracking prevention may cause Cu discoloration when added in large amounts, resulting in undesirable appearances. Further, in addition to cracking, minute Cu surface roughness may also occur.
Another problem is that the Cu surface damaged by the dry process can be readily oxidized. Therefore, when a wafer is exposed to air during transfer from one process to another after a chemical treatment using a polymer-removing solution or the like, an oxide film easily grows on the surface of the Cu metal interconnect. This Cu oxide film can also cause defects in semiconductor devices, often leading to defective devices. The Cu oxide film can be removed by argon sputtering, hydrogen reduction, and the like. However, argon sputtering often damages Cu surfaces, and hydrogen reduction can cause cracking along the grain boundaries of Cu surfaces. It is thus important to prevent the growth of a Cu oxide film.
Patent Document 1, for example, discloses using, in the post-CMP cleaning step, an anticorrosive such as benzotriazole at the same time as or subsequent to the removal of metal contaminants using a cleaning solution containing a carboxylic acid compound such as oxalic acid.
Benzotriazole, however, has the disadvantage of exhibiting only a small Cu antioxidant effect, and having large adverse effects on the environment due to its poor degradability. Moreover, although Patent Document 1 discloses indazole as an example of the anticorrosive, it does not disclose a specific chemical solution or treatment conditions. Further, Patent Document 1 discloses indazole as an example of four-membered heterocyclic compounds. This disclosure, however, is obviously technically incorrect.
Furthermore, Patent Document 1 teaches in paragraph
the use of an aqueous solution containing 0.01 to 1% oxalic acid (a cleaning solution). However, the aqueous solution of oxalic acid at this concentration has a pH of 1.5 to 3, which is lower than the pKa of oxalic acid (3.82). The Cu antioxidant effect is therefore small, resulting in Cu surface cracking or roughness.
Accordingly, there is a demand for a residue-removing solution for use after a dry process that serves not only to prevent Cu surface oxidation, but also to prevent Cu surface cracking and roughness. No such residue-removing solutions, however, have yet been developed.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-148385